Our long range goal is to understand the impact of dietary consistency on the aging oral lining tissues. The present proposal specifies an ancillary study designed to serve as a baseline for a major investigation on aged rats, for which a pending renewal application has been submitted to USPH. Studies on bone and salivary glands have demonstrated that disuse (immobilization of bones, elimination of mastication due to liquid diets) engenders an atrophic process, which in human bones progresses for as much as 6 months and leads to dangerously increased fragility. It is then followed by reduced, but balanced bone formation and resorption. We have recently shown the occurrence of atrophy in oral epithelium of rats placed on a liquid diet at 4 months of age. Five weeks on this regime produced measurable epithelial atrophy and a drop to half or less of the mitotic rate. The purpose of the proposed study is to determine in rats on a liquified diet (1) the duration of the process of progressive atrophy, and (2) the status of oral epithelium at this point in time. 1. Thickness of cellular and keratin layers and the number of rows in each layer will be determined in rats kept on a liquified diet for increasing periods of time. The endpoint of progressive atrophy will be manifest when two successive periods no longer show progressive thinning. The shorter of the two periods then indicates the duration of the atrophic process. 2. The status of the mucosa at this time and its prospective fate will be judged on the basis of the degree and nature of thinning, and the degree of slowing of cell division and cell migration, established with the use of labelled thymidine at 1 hr. and 3 days after injection. Our experience in dealing with lesions of the oral soft tissues should enable us to evaluate prospective ulceration and impairment of impermeability from this status assessment.